1. Field of the Invention
This invention relates generally to a method for producing diamond by chemical vapor deposition.
2. Description of the Prior Art
Diamond has long been sought after not only for its intrinsic beauty and value as a gemstone but for its many unique and valuable mechanical, electrical, optical, and thermal properties. Diamond is the hardest material occurring in nature, has a low coefficient of friction, and is extremely resistant to chemical attack. It also is classified as a high bandgap semiconductor, is optically transparent to much of the electromagnetic spectrum, and has the highest heat conductivity of any material at room temperature. While, in fact, naturally occurring diamonds are far from scarce, humankind has long sought to produce these cystals synthetically.
The first such syntheses involved the application of high pressures and temperatures to bring about the allotropic transformation of graphite to diamond (for example, see U.S. Pat. No. 2,947,608). More recently, numerous studies have shown that diamond can be produced sythetically at low pressures using various fores of chemical vapor deposition (CVD) processes employing a gaseous carbon compound (for example, see U.S. Pat. Nos. 3,030,187, 4,767,608, and 4,873,115).
In all of the CVD processes to date, with the exception of CVD growth on diamond single cystals where the growth is epitaxial for thin layers, the diamond produced takes the form of a thin polycrystalline film of extremely small diamond particles (typically less than 100 micrometers in diameter). In many of the processes, diamond is not formed by itself but rather in combination with graphite and diamond-like carbon (the latter species being a carbon allotrope with properties between those of graphite and diamond). The prior art processes for producing diamond all employ some high energy method of pretreating or activating one or more of the reactant species such as microwave or rf-generated plasmas (plasmas being the mixture of electrons and gaseous ions formed when the gases are heated to the range of 5,000.degree. C. to 30,000.degree. C.), or hot filaments, high temperature flames, arc discharges, electron beams, lasers, etc. (which heat the gases to a temperature of 2,000.degree. C. to 3,000.degree. C. or higher). All such high-temperature, high-energy steps comprise methods of pre-treatment of said gases to activate them to a "high energy level", by which is meant heating of the reactant gases to a temperature of 2,000.degree. C. or higher. The activated gases then are impinged upon a substrate, with diamond growth occurring principally on the substrate surfaces directly in the path of the activated gases or plasma. Such processes are expensive because of the energy costs of activating the reactant gases and are relatively low-volume because of the difficulties of activating large volumes of gases. Also, they make it difficult to coat three dimensional or irregular objects with diamond film, because the objects must be turned to expose successive sides or areas to the flow from the activated gases.
All the CVD methods developed to date are physically-activated processes where atomic hydrogen, essential for diamond CVD, is first generated from the direct dissociation of molecular hydrogen. Because of the strong chemical bond in molecular hydrogen, the dissociation of molecular hydrogen requires very high activation gas temperatures and hence high activation energies. Currently, there are five major CVD diamond processes, namely, hot filament, combustion, microwave plasma, RF plasma, and plasma arc deposition. All of these processes involve gas temperatures greater than 2,000.degree. C. in order to generate atomic hydrogen. The active carbon-containing species necessary for diamond CVD are generated either from the reactions of atomic hydrogen with the corresponding carbon-containing precursors, or from a direct activation by physical means such as plasmas and thermal heating. The activated gases then are impinged upon a substrate which, with the exception of diamond single crystals as substrates, is usually seeded with extremely small diamond particles by mechanical means such as rubbing and sonicating. Such processes depending on the particular process are expensive because of energy, materials, and capital costs of activating the reactant gases.
In hot filament CVD one has to consider the possible incorporation of the filament metal into diamond. Also in the hot filament and especially the plasma activation methods, one is likely to create active nitrogen species which are then responsible for nitrogen incorporation in diamond. Nitrogen impurities are known to affect electrical, thermal and optical properties of diamond.
Another major drawback to presently existing CVD processes is the requirement of high deposition or substrate temperatures (typically &gt;700.degree. C.). The growth of diamond at high substrate temperatures typically heightens the strain by thermal expansion differences between diamond and the substrate. The requirement of high substrate temperatures also excludes the direct coating of low temperature materials such as low melting ceramics, glasses and metals which do not withstand high temperature environments. Our invention eliminates many of these problems.
Most CVD processes also occur at pressures less than 100 Torr; those which do not, typically produce very impure diamond/graphite mixtures.
Many potential markets exist for diamond films and may involve the use of diamond coatings for extreme hardness, inertness to chemical attack, heat conductance, and other desirable properties. Some applications may further use doped diamond for its unique electrical properties. A major drawback to presently existing CVD diamond coating technologies is the difficulty of placing a diamond fill uniformly on objects with complex shapes. Another major problem exists with the high temperatures (typically &gt;700.degree. C.) usually required for diamond formation in existing CVD processes. Our invention eliminates many of these problems.
In U.S. Pat. No. 5, 071,677 there is described a method for depositing CVD diamond films and particles employing a halogen-assisted technique. While this technique is unique and solves many of the problems associated with CVD diamond deposition, it has been found that diamond deposition may be enhanced further by the addition of a chalcogen to the process of U.S. Pat. No. 5,071,677.
Chalcogens comprise the elements oxygen, sulfur, selenium, tellurium and polonium from group VI of the periodic table. As will be appreciated by those skilled in the art, a chalcogen (oxygen) may be present in minor amounts as an impurity in processes such as those disclosed in U.S. Pat. No. 5,071,677, either as a feedstock impurity or if elaborate steps are not taken to exclude and fully purge ambient air from the reactor, tubing, etc. Additionally, example 1 of the application used as a feedstock natural gas which contains minor amounts of oxygen as an impurity.
Subsequent to the filing of the application for U.S. Pat. No. 5,071,677, some researchers, attempting to duplicate the results there disclosed, have experienced difficulty in producing pure diamond films, if they took steps to exclude and purge all air from the reactor system. Other researchers, using similar oxygen-excluding measures, have reported no difficulty in duplicating applicant's results.
As a result of further study and experimentation, it was determined that the production of substantially pure diamond films, in accordance with the methods of U.S. Pat. No. 5,071,677 may be accomplished most reliably by the inclusion in the reactant gases of a minor amount of a chalcogen, most preferably oxygen or sulfur. The incorporation of a chalcogen into the process of U.S. Pat. No. 5,071,677, was the principal feature of Continuation-In-Part Application Ser. No. 07/696,769, now U.S. Pat. No. 5,316,795.